The present invention relates generally to optical semiconductor devices, and more specifically, the present invention relates to a photodiode with low dark current.
Optical communication systems utilize light energy to carry information. To interface optical communications with electronic devices, the information contained in the light energy must be extracted and converted into an electrical signal. Optical-to-electrical conversion circuits, such as photo-receiver circuits, provide this conversion.
Photo-receiver circuits commonly utilize photodiodes. Photodiodes are semiconductor devices that adjust their conductance (or resistance) in accordance with the amount and type of light energy they receive. Ideally, when a photodiode absorbs a relatively high amount of light, it conducts electricity freely, and when a photodiode absorbs relatively little light, it conducts electricity poorly. Photodiodes are typically combined with electrical signal amplification circuitry, such as a heterojunction bipolar transistor (xe2x80x9cHBTxe2x80x9d) amplifier, to amplify an electrical signal produced using the photodiode. Thus, the information carried in the light energy is converted to corresponding information in an electrical signal.
Present photodiodes, however, are not ideal. In particular, they suffer from a non-zero conductance when receiving no light energy. In other words, even when the photodiode is exposed to no light energy, present photodiodes allow a small current to flow. The current that flows when the photodiode is not exposed to light energy is referred to as the dark current. Hence, a photo-receiver incorporating photodiodes produces an output electrical signal, substantially comprising noise, in the absence of input light energy.
Several techniques have been tried in the past to reduce the dark current. As one example, dielectric materials, such as silicon dioxide or silicon nitride, were used for surface passivation. In the surface passivation process, the exposed surfaces of the photodiode are coated with the dielectric material to protect the semiconductor surface and to make the surface less electrically active.
A problem with many surface passivation techniques is that they are not generally compatible with the HBT production process. For example, passivating with organic materials, such as polymide, has been found to reduce dark current. However, passivating with such organic materials typically involves curing the materials at high temperatures. The high temperatures may, in turn, damage the surface or the bulk of the HBT devices, resulting in electrical leakage, doping diffusion, and ohmic contact degradation. Another problem with some of the organic passivation materials is that they are hydroscopic and absorb moisture. This moisture, in turn, adversely affects the performance and reliability of the underlying semiconductor devices.
Additional examples of dark current reduction techniques include altering the deposition process used during passivation and performing a special surface cleaning using acid. In general, however, past dark current reduction have focused on treating the surface of the semiconductor material in some way. All of these techniques fail to address the contribution to dark current inherent in the fumdamental structure of the photodiode itself.
Thus, a need has long existed for a photodiode with low dark current. A need has also long existed for a method for reducing dark current in a photodiode that is compatible with the HBT production process.
Accordingly, it is an object of the present invention to provide a low dark current photodiode. It is also an object of the present invention to provide a method for reducing dark current in a photodiode. It is a further object of the present invention to provide a method for reducing dark current in a photodiode that is compatible with the HBT production process.
One or more of the foregoing objects are met in whole or in part by a preferred embodiment of the present invention that provides a photodiode comprising an absorption layer and a barrier layer. The barrier layer comprises a barrier layer material having a wider band-gap than the band-gap of the absorption layer material. The barrier layer comprises sublayers, which are doped to position the high-electric field region of the photodiode in the barrier layer. Positioning the high-field region of the photodiode in the wide band-gap barrier layer and away from the narrow band-gap absorption layer substantially reduces the flow of dark current through the photodiode.
A preferred embodiment of the present invention provides a method for fabricating a low dark current photodiode. The method comprises building a barrier layer into the structure of a photodiode. Building the barrier layer includes building a layer of semiconductor material with a wider band-gap than the absorption layer material and doping the barrier layer material to position the high-field region of the photodiode in the barrier layer, thus reducing dark current.